The invention relates generally to guy line supported marine structures and in particular to guy wire supported marine structures designed to operate in water having a depth greater than about 300 feet.
Over the past thirty years, fixed jacket-type platforms have represented the most common structural solution for providing drilling and production facilities in water based structures. As the need to move into deeper waters arose, technological advancement, ever growing expertise, more sophisticated analysis techniques, and faster and larger computers pushed the state of the art further and further. Today, the tallest jacket structure stands in 1,050 feet of water in the Gulf of Mexico.
However, there are several indications that with today's present technology, water depths beyond about 1,050 feet may require an altogether different approach. One of the main problems faced by the designer of a deep water marine structure is the dynamic interaction between waves and the structure. In shallow water, for example 300 feet, a typical jacket has a natural period of about 2 seconds; and this period is much smaller than the peak periods of the various sea states which are typical of, for example, the Gulf of Mexico. Accordingly, the ratio of the structural period of the dominant wave period (in the Gulf of Mexico) is less than one and typically this corresponds to a point on a dynamic amplification factor (DAF) curve which is to the left of the resonance peak. Thus, as long as the period ratio is small enough the dynamic amplification factor is close to one and the structural response is essentially static. However, as the water depth increases, the structural period increases and approaches the spectral peaks for the body of water; thus the dynamic amplification factor increases, becomes larger than one, and moves closer to the resonance peak. For example, for a 1000 foot jacket, the structural period is 4-6 seconds. Under this circumstance, in the Gulf of Mexico, the interaction of the jacket with a design storm is limited but the energy associated with an operating sea state is amplified significantly . As a consequence, fatigue becomes a critical aspect of the design and modification may be needed to stiffen the structure. This results in a dramatic increase in the required steel tonnage, in additional costs, and in fabrication and installation problems.
Consequently, workers in the art have truned to a different approach. Rather than trying to minimize the dynamic wave-structure interaction by reducing the structural period, the same effect was obtained by making the structural period larger than the wave period. The so-called compliant structures which resulted from this approach were the guyed tower platform, the tension leg platform, the buoyant tower, etc. A common characteristic of these structural approaches is that the ratio of the sway period to the wave period is greater than one and accordingly the dynamic amplification factor is less than one, thereby reducing dynamic loads.
With respect in particular to the guyed tower platform concept, the main areas of emphasis have been to provide sufficient compliancy to enable the structure to oscillate with the waves without over stressing the foundation. Prior art references describe for example the adaptation of the spud can as a foundation solution. In addition, the upper portion of the marine structure is guyed by using a clump weight/anchor structure configuration. This system is described in U.S. Pat. No. 3,903,705, which was issued on Sept. 9, 1975.
The structure described in U.S. Pat. No. 3,903,705, however requires significant offshore installation time, provides minimum control over clump weight spinning, and provides difficult accessibility for pipelines between the guy line structure when the number of guy lines increases (as will occur as the depth of the water increases). In addition, the spud can described in U.S. Pat. No. 3,903,705 has several disadvantages.
It is therefore an object of this invention to provide a guyed marine structure and method providing more control over the clump weight, better accessibility for pipelines between guy lines, and a significant reduction in offshore installation time for the mooring guy lines. Other objects of the invention are to provide a marine structure and method wherein the cost of the marine structure for deep water depths is less than that of a comparable jacket-type platform.